Enteroviruses are characterized by their high replication rates and extreme sequence plasficity, which allow them to rapidly adapt to different environments and hosts. These viruses depend entirely on the host protein homeostasis machinery, composed of molecular chaperones and quality control (QC) components such as the ubiquitin-proteasome system, for viral protein production and function. Enterovirus replication poses several challenges to the cellular protein homeostasis machinery as the need to produce high amounts of protein in a very short time places a big burden for the host protein production and folding machineries. Furthermore, enteroviral proteins tend to be large, complex and multifunctional, and thus likely to require the assistance of molecular chaperones to fold. Indeed, we have shown that the Hsp90 chaperone system is essential for capsid folding and assembly for many, perhaps most, picornaviruses, including the enterovirus polio- and coxsakie-viruses. Since other aspects of enterovirus replication Involve additional large multlprotein complexes, chaperones are likely to be broadly required for other aspects of the viral cycle. An important challenge to protein homeostasis in RNA viruses arises from their very high mutation rates, which pose a big burden to viral protein stability and are likely to produce high levels of non-functional or destabilized proteins. These mutant proteins must be either maintained in a funcfional state or eliminated from the cell to prevent dominant negative effects on viral function. We hypothesize that these functions are carried out by chaperones, which can buffer metastable proteins, as well as by the ubiquitin-proteasome system, which targets misfolded proteins for degradation. To understand the molecular and cellular mechanisms by which cellular chaperone and quality control machineries control viral protein homeostasis, and allow the virus to replicate we propose the following Aims: Aim 1: Define the chaperone components required for enterovirus replication. Aim 2: Define the role of the Quality control (QC) machinery in picornavirus replication Aim 3: Examine the plasticity and interplay of chaperone and QC pathways during viral infection.